Copying model decals as vector graphics
I agree with "10 Replies": Yes 

A vector recreation, saved as high of a resolution png that you need,
can be much better than using just a scan.  (Unless it's a good scan already, but you'll probably want to cut it out, etc...  Still a vector will be noiseless and sharp at any resolution.  An exception is if your decals have gradients or many colors-- if it's more like a photo than line art/ clean sharp shapes.)

As mentioned already too, you can't use vectors directly as a texture in Cycles.  (But you could at least with the old renderer: https://www.blender.org/forum/viewtopic.php?t=12870 )

But I also agree with Duarte's "Don't bother retracing" comment.

Instead, use a tool like Illustrator's Live Trace, Inkscape's Trace Bitmap, or http://vectormagic.com to save a ton of time.  Then you can save your clean, sharp vector as a raster texture to size the you need.  Later, if needed, you can make an even higher resolution png texture for your blender render.

A good clean vector file of vector-like art is always preferred to have, even if you can't use it directly as a texture in Blender.